Automatic Cooling, Vanilla Style!

 how to connect a twelve volt fan to the DS1620

What did you think, that I wanted to scratch my brain for a while and figure it all out on my own? Dallas Semiconductor provides this great schematic drawing to show how the wires hook up to its DS1620 thermostat/thermometer IC. This drawing was blatantly ripped from their site because I wouldn’t dream of redoing it when I can tell they put enough effort into it already!

You can see it’s all really simple. The 2N7000 field-effect transistor (FET) is key to making it all work smoothly. A little tiny computer chip like the DS1620 just isn’t designed to pass a lot of current. The 100mA fan they suggest in the drawing would likely melt down the IC even if you cooled it with ice water. What the FET does is pass all the current required to operate the fan—or in my case, two fans. To be safe, I actually used one 2N7000 for each fan in my circuit.

The FET in this circuit turns on the fan when the DS1620 puts a voltage on its gate . Pin 5 of the DS1620 is shown connected directly to the gate of the FET.

Inside the FET, what is happening is this voltage (5V) is creating a magnetic field that entices current to flow through it and into the fan, hence the name field-effect transistor. You can imagine a glass full of water resting on an uneven table top. The water is just about pouring over the edge at the top of the glass, so close that you can see it standing on the rim, in fact. But the water will not move until you break the surface tension with some object or force. This is similar to the FET and how magnetic fields work to cause current to flow in it. The power channel is opened by the applied magnetic field.

No Charge Air & Automatic If You Act Now!

The automatic part of this cooling system comes in with a combination of hardware (the DS1620 IC) and software. The software is free from Dallas Semiconductor. It’s a demo piece of software written in C for DOS, but it runs under all iterations of Windows at least to Win98SE as well. I found it on the Dallas Semiconductor public FTP site.

Baa-aa-aa-aa-aa, Baby!

If you’re running Windows in the dumbed-down style, which is done by default on all later versions, you won’t see the file as demo.exe but rather just plain demo . Don’t be sheepish, it’s not really your fault! This disastrous presumption by Microsoft assumes that all its users are inept and should not be allowed to see anything but pretty pictures with descriptive names beside them. To fix this problem—and possibly open up a whole new world for yourself—double-click on the My Computer icon, then click on View at the top of that window, and finally click on Folder Options to bring up the appropriate menu. Note that these instructions are for Windows 98 specifically, so you may have to dig around in similar spots to get here.

Now, you’re in the Folder Options menu. Click on the View tab and look under the Advanced Settings window that appears. There will be a couple of choices under what looks to be a folder called Hidden Files . Click on the Show all files option, and right below that, ensure that the box is unchecked beside the Hide files for known file types option.

I warn you now, if you’re not familiar with these settings and what they will do,the #1 rule from this point on is DO NOT ERASE files or folders that you see if you do not know what they are or how they got there. You may very well end up losing your whole computer to the netherworld if you start throwing all sorts of things into the Recycle Bin just because you have never seen them before! The safe thing to do is leave it alone unless you created it.

Getting On With It

From here you’ll also need to know how to attach the DS1620 IC to your parallel port so this demo piece of software can set the temperature limits and show you the current temperature of the chip, in case you care. For that little ditty I poked around a bit on Dallas’ FTP site.

You see, Dallas Semiconductor is an awesome company for simply doing things right. Along with providing you a typical application for their products (in our case the fan control circuit using their DS1620 chip), they also make available the necessary software (if any) and with it they will often provide the source code. The source code for any program written in C is simply a text file. This is a bonus, because now we as users of the software can see exactly how it’s all done—if we understand how to program in C, that is. If you, like me, could care less about programming in C or any other computer language, don’t think that this all means nothing to you!

As the tech savvy individual you are, you should know that source code files for demo software can provide valuable information beyond just the normal “this line does this” type of commentary. As a prime example, you can download the Appnote 105 source code from Dallas’ FTP site and open up Threewir.h in your favourite text editor. Right at the top of the file you see this:

This file defines the method for setting the 3-wire interface lines from an
IBM-PC compatible computer. The interface is through the parallel port.
The parallel port lines used are as follows:

/SELIN pin 17 DQ
Data4 pin 5 CLK
Data3 pin 4 /RST
GND pin 18 GND

Voila, instant hookup chart telling you which lines you need to connect from your parallel port (printer port) to which pins on the DS1620 IC! I tell ya, the things you can get for free if you’re tech savvy are gonna make you happy as a pig in… mud!